1. Field of the Invention
The present invention relates to a pull roll assembly used in the production of a glass sheet made by the fusion process and, in particular, to techniques for controlling the operation of the pull roll assembly based on pulling forces associated with the glass sheet as it is being drawn by the pull roll assembly.
2. Description of Related Art
Corning Inc. has developed a process known as the fusion process (e.g., downdraw process) to form high quality thin glass sheets that can be used in a variety of devices like flat panel displays. The fusion process is the preferred technique for producing glass sheets used in flat panel displays because the fusion process produces glass sheets whose surfaces have superior flatness and smoothness compared to glass sheets produced by other methods. The fusion process is described in U.S. Pat. Nos. 3,338,696 and 3,682,609, the contents of which are incorporated herein by reference.
The fusion process utilizes a pull roll assembly (e.g., fusion draw machine) that draws the glass sheet between two rolls to stretch the glass sheet to a desired thickness. In particular, the pull roll assembly has two rolls that directly contact the glass sheet and exert a pulling/drawing force on the glass sheet. The final thickness of the glass sheet is inversely related to the pulling force (required sheet pulling force) applied to the glass sheet when the glass sheet is drawn between the two rolls. As such, to stretch the glass sheet to a desired thickness one needs to maintain the required sheet pulling force on the drawn glass sheet.
In the traditional pull roll assembly, the pulling force is indirectly maintained by precisely controlling the speed of rotation with which the rolls act on and rotate against the glass sheet. This indirect means of maintaining the pulling force can lead to a situation where the two rolls damage the surface of the glass sheet. The damage to the surface of the glass sheet is caused when only the rotational speeds of the two rolls are controlled which means that one of the two rolls is going to always slide on the glass sheet because it is impossible to have two rolls with the exact same diameter. The slippage of the rolls not only damages the surface of the glass sheet but can also damage the rolls themselves and break the glass sheets resulting in manufacturing downtime. Accordingly, there is a need for a pull roll assembly that addresses this and other shortcomings associated with the traditional pull roll assembly. This need and other needs are satisfied by the pull roll assembly of the present invention.